In an engine mounted on a vehicle, there may be caused misfires due to unstable combustion of the fuel in a combustion chamber due to various factors such as lean mixture of the air-fuel ratio by driving under lean combustion, a malfunction of ignition systems or fuel systems, and a failure of components of the engine. When misfire of the engine occurs, unburned gas flows to the catalyst in the exhaust system and burns which causes a rise in catalyst temperature that detriments the performance or damages the catalyst. For this reason, some engines are provided with misfire detectors to detect the misfire and control the fuel.
Examples of prior detectors are disclosed in the following references: JP Laid-Open No. H05-10180; JP Laid-Open No. H09-217642; JP Laid-Open No. 2000-265893; and JP Laid-Open No. 2002-138894.
In prior misfire detectors for the multi-cylinder engine, “fuel feedback control prevention” and “fuel cut” are chosen based on the rate of misfire so as to protect the catalyst from the high temperature or damage (for example, see JP Laid-Open No. H05-10180).
Also, there is another prior misfire detector for the multi-cylinder engine in which fuel feedback control is stopped based on the misfire rate and the fuel cut, or halt of fuel supply, is performed to protect the catalyst, and the driver is informed of short of fuel due to the misfire so as to reduce cost for repair (for example, see JP Laid-Open No. 2002-138894).
Further, there is fuel control while the fuel cut is performed to protect the catalyst.
Here, control of the prior misfire detector for the multi-cylinder engine is explained below. Referring to FIG. 13, a control program for the misfire detector for the multi-cylinder engine starts in step 502. Rate of misfire is calculated for each cylinder in step 504. Then a determination is made in step 506 as to whether the rate of misfire is more than or equal to a criteria A by which to judge whether the catalyst may be damaged.
Rate of misfire is calculated for every 200 engine revolutions for each cylinder, including the cylinder for which the fuel cut is performed due to the misfire. For a V-type engine, the rate of misfire is calculated for each cylinder bank. The rate of misfire is determined from a table depending on the engine speed and the engine load.
The determination is made in step 506 as to whether the misfire rate is more than or equal to the damaging criteria A. In this step, the cylinder for which the fuel cut should be performed (fuel cut cylinder) is determined through a determination process for the fuel cut cylinder as shown in FIG. 14. In order to determine the fuel cut cylinder, a determination program starts in step 602. In step 604, the misfire rate for each cylinder is calculated by dividing the number of misfires by the number of ignitions (for every 200 revolutions of engine speed). Then a determination is made in step 606 as to whether the rate of misfire is more than or equal to the damaging misfire criteria A. Further, another determination is made in step 608 as to whether the number of misfires for each cylinder SIKCYLi (wherein i is a cylinder number) is greater than a set rate B (e.g. 30%) of the number of misfires of all cylinders for every 200 revolutions of engine speed.
If both of the determinations in steps 606 and 608 are “YES”, then the misfire cylinder is determined in step 610 and the determination process ends in step 612. If either the determination in step 606 or 608 is “NO”, then the process returns to step 604.
If the misfire cylinder is determined and the determination in step 506 is “YES”, a fuel feed back control (F/B) for the fuel cut cylinder is halted and is initialized in step 508. In this initialization of the fuel feed back control, the correction value is set at zero or a default value.
Then, in step 510 a learning control to learn the quantity of fuel feed back for the cylinder for which the fuel cut should be performed is prevented. The correction value of learning is initialized to an unlearned state in step 512. In this initialization of the learning control correction value, the correction value is set at zero or a default value. Also, the learning control is set at the initial state as in the state at start of the engine or the state at which a battery is changed.
Then the fuel cut for the misfire cylinder is performed in step 514. Increase in the fuel supply to the cylinders for which the fuel cut is not performed is prevented in step 516, and the process returns to step 504. In this fuel increase prevention process, the fuel increases for acceleration, enrichment, and other conditions are prevented.
In the calculation of the misfire rate after the implement of the fuel cut for the misfire cylinder in step 514, the calculation of the misfire rate in step 504 includes the cylinder for which the fuel cut is performed, and a determination is made in step 506 as to whether the calculated misfire rate is more than or equal to the criteria A by which to judge whether the catalyst may be damaged.
If the determination in step 506 is “YES”, the processes in steps 508–516 are performed. In contrast, if the determination in step 506 is “NO”, the effect of the misfire on the catalyst is not significant, so that the fuel cut for the misfire cylinder is canceled in step 518, the fuel feed back control is resumed in step 520, the learning control is resumed in step 522, the fuel increase control prevention is canceled in step 524, and the process returns to step 504.
At this time, the process in step 518 to cancel the fuel cut for the misfire cylinder may be replaced by a condition based on set engine speed and set engine load, instead of the damaging criteria A (FIG. 5).
According to the above-mentioned prior control, the fuel cut control, the fuel feed back prevention control, the fuel learning prevention control, and the fuel increase prevention control are performed when misfire occurs to prevent a rise in the catalyst temperature. However, the air-fuel (A/F) ratio becomes lean and results in undesirable engine knocking, as shown in a time chart in FIG. 15. At this time, no ignition timing control is performed to prevent the knocking.
Moreover, even if the fuel cut control, the fuel feed back prevention control, the fuel learning prevention control, and the fuel increase prevention control are performed when the misfire occurs, the vehicle speed can be increased by depressing a throttle pedal in case the misfire frequency is low or the engine has a large piston displacement. This results in not only a bad influence on the engine and the catalyst but also emission of the exhaust gas in a large amount in the ambient, since the normal appropriate control for exhaust gas is not performed.